Clutches are commonly utilized in unidirectional drive systems for transmitting drive torque from a drive shaft to a driven shaft. For example, starters of the type commonly used to start engines, in particular the turbine engines of modern gas turbine powered aircraft, often employ a pawl and ratchet type clutch which functions to transmit rotational drive torque from a drive shaft of the starter to drive the engine being started to starting speed. One type of starter often employing a pawl and ratchet clutch is the pneumatic starter, also known as an air turbine starter, such as disclosed, for example, in U.S. Pat. Nos. 3,727,733; 4,899,534; 4,914,906; and 4,926,631.
A pawl and ratchet clutch of the type commonly used in such pneumatic starters includes a toothed ratchet member mounted on a central drive shaft and a plurality of pivotal pawls supported from and rotating with a driven output shaft disposed coaxially about the drive shaft. The pawls are operatively disposed at circumferentially spaced intervals about the ratchet member in cooperative relationship therewith. Each pawl is biased to pivot radially inwardly by a leaf spring operatively associated therewith to engage a tooth of the ratchet member thereby coupling the drive shaft in driving relationship to the driven output shaft so long as the pawls remain engaged with the teeth of the ratchet member. The drive shaft is connected, either directly or through suitable reduction gearing as desired, to the shaft of the pneumatic starter turbine, which is powered by extracting energy from a flow of pressurized fluid passed through the turbine of the starter.
To start the turbine engine, the output end of the driven output shaft of the starter is connected, for example by mating splines, to an engine shaft operatively connected to the main engine shaft through a gear box, and pressurized fluid, typically compressed air, is passed through the turbine of the pneumatic starter. As the starter turbine extracts energy from the compressed air passing therethrough, the drive shaft of the starter turbine is rotated to in turn rotatably drive the output shaft of the starter, and consequently the turbine engine shaft connected thereto, through the engagement of the pawls pivotally mounted to the output shaft with the ratchet member mounted to the drive shaft. Typically, the starter is designed to accelerate the engine shaft from zero to a predetermined cut-off speed, typically of about 5000 revolutions per minute, in about one minute or less.
Once engine light-off has occurred and the engine shaft is rotating at the desired cut-off speed, the flow of pressurized air to the starter turbine is terminated. With the flow of pressurized air to the starter turbine shut-off, the drive shaft of the starter rapidly slows down. Consequently, the ratchet member mounted to the starter drive shaft also rapidly slows down, while the pawls supported from the starter output shaft continue to rotate with the shaft of the operating turbine engine at the relatively high cut-off speed. The pawls become disengaged from the ratchet member when the rotational speed of the output shaft exceeds a threshold speed whereat the pawls lift-off of the ratchet member, that is pivot radially outwardly out of contact with the teeth of the ratchet member, under the influence of the centrifugal forces acting thereon due to the continued rotation of the pawls at the relatively high speed of the engine shaft. The pawls remain disengaged from the ratchet member so long as the rotational speed of the engine shaft remains high enough that the centrifugal forces acting on the pawls exceed the opposing moment imposed on the pawls by the force of the bias springs.
When the turbine engine is later shut-down, the operating speed of the engine shaft of the turbine engine to which the output shaft of the starter is connected decreases as the turbine engine spools down. As the starter output shaft slows down, the centrifugal force on the pawls consequently decreases and the force of each bias spring progressively pivots its associated pawl radially inwardly again toward the ratchet member until each pawl again contacts the ratchet teeth on the nonrotating ratchet member so as to ratchet in a position to allow for reengagement of the clutch. The speed at which the reengagement of the pawls with the ratchet member occurs, commonly referred to as the reengagement speed, is less than the pawl lift-off speed by an amount commonly referred to as the clutch hysteresis.
In pawl and ratchet-type clutches, when reengagement occurs with the ratchet member rotating at too high of a rotational speed relative to the pawls, a crash reengagement occurs which often results in substantial, if not catastrophic, damage to the clutch. Crash reengagement generally takes place when following an aborted turbine start occurring after the turbine has exceeded pawl liftoff speed, the starter is reactivated in an attempt to restart the turbine engine, before the shaft of the turbine engine has spooled down to a speed below that at which pawl reengagement occurs. When air flow to the starter is reinitiated without the pawls and ratchet being engaged, there is no load on the starter and, consequently, the starter shaft and the ratchet member mounted thereto rapidly accelerate to a free running speed which is substantially above the pawl reengagement speed. As the turbine engine continues to spool down to the pawl reengagement speed, the pawls will eventually pivot inwardly, as the centrifugal force urging them outwardly deteriorates, until the pawls recontact the ratchet member. With the ratchet member rotating at a free running speed substantially greater than the speed at which the pawls are rotating, a violent reengagement will take place. To avoid such a crash reengagement, it is customary to delay restart of the turbine engine until it is certain that the turbine engine has slowed down to the point at which the pawls have already reengaged that ratchet member before the starter is reactivated by supplying air flow to the starter turbine.
British Patent No. 866,046 discloses another form of aircraft turbine engine and starter turbine clutch engagement assembly which utilizes radially displaceable balls to both urge engagement and disengagement between the members of the clutch assembly on the engine and starter turbine. The displaceable balls move over complex cam surfaces each having two angularly related portions with different angles of inclination. A spring is also included to bias the clutch members into engagement when the starter turbine and turbine engine are at rest. When the starter turbine is started, the engagement balls will move radially out to a clutch-engagement locking position which will effectively lock the starter turbine clutch member to the turbine engine clutch member. This clutch-locked condition will persist until the turbine engine rotation speed exceeds the starter turbine rotation speed due to ignition of the turbine engine. When the turbine engine begins rotation, the disengaging balls will also be thrust radially outwardly, but they will not be able to cause disengagement of the clutch members so long as the turbine engine and starter turbine are rotating at the same speed because of the angle of inclination of the cam surface engaged by the engaging balls. Once the turbine engine speed exceeds the starter turbine speed, the turbine engine clutch component will begin to ratchet over the starter turbine clutch component. This ratcheting action will force the starter turbine clutch component away from the turbine engine clutch component, thus compressing the engagement spring. At this point in time, the engagement balls will be forced out of the engagement position, and the disengagement balls will be able to move out to their disengagement position on their cam surface, whereupon the clutch members will be held in a disengaged relationship. When both engaging and disengaging balls are at the same radius their forces are balanced. When the jaws are engaged, the engaging balls are at a greater radius and therefore exert an engaging force greater than the disengaging force produced by the disengaging balls. The compound cam tracks only increase the axial force at any given rotational speed a feature meant to reduce ratcheting as the engine coasts down to a standstill. The use of the compound cam tracks disclosed in the British reference requires ratcheting action to cause disengagement of the clutch members. Thus, so long as ratcheting is ongoing, the engagement balls will essentially null each other and will not be able to assist in forcing the starter turbine clutch member into engagement with the turbine engine clutch member. This does not create a problem when the turbine engine and starter turbine are started from a rest position. As soon as the differential speed of the engaging balls drops off, the force produced by the disengaging balls will overcome the force of the engaging balls plus the engaging spring force and free running will be achieved.
In the assembly described in the aforesaid British patent, there are an equal number of engagement balls and disengagement balls. In the disengaged position both sets of balls are positioned at the same radius. As long as the disengaging balls are rotating at a greater speed than the engaging balls they will exert more force than the engaging balls. At synchronous speed the ball forces will be balanced. Shortly before synchronous speed is reached the spring will provide the force to start clutch engagement. As soon as axial motion occurs the engaging balls will move to a larger radius and the disengaging ball will move smaller radius. This action causes a snap action to bring the jaws into ratcheting engagement shortly before synchronous speed is achieved. When the starter turbine is restarted as the turbine engine decelerates after flame out, the disengagement balls will be in their outwardmost position so long as the turbine engine is rotating faster than the starter turbine. Reengagement will thus be resisted. Since the mass of the engagement balls is the same as the mass of the disengagement balls, so long as the starter turbine speed is lower than the turbine engine speed, the engagement balls will not be able to force the disengagement balls out of their disengaging positions to allow preengagement ratcheting to begin. At rest the spring holds the clutch in engagement and the ball centrifugal forces are zero. As speed is increased the engaging balls move to a radial position greater than the disengaging balls and add to the engaging force of the spring.
Thus in the British patent clutch assembly, preengagement ratcheting cannot begin until the rotational speed of both the starter turbine and the turbine engine are essentially equal. This result was intentional, and was sought so as to minimize preengagement ratcheting, which was thought to be harmful to the clutch members. Thus a very brief period of ratcheting occurs. This can create a problem in the event that there is some ambient resistance to movement of the clutch members into reengagement, such as dirt, lubrication gum, or the like. In the latter case, a late reengagement can occur which can harm the teeth on the clutch members.
Another problem that occurs with a clutch system of the type shown in the British patent relates to the occurrence of only partial reengagement between the clutch member teeth. The teeth on the clutch members are cut at a 10.degree. angle to the face of the members for ease of manufacturing. In the British patent, there is described a spline assembly which is formed at an angle that is complimentary to the angle of the teeth to prevent disengagement of the teeth when the starter turbine clutch member moves toward the turbine engine clutch member. When the teeth angle and the spline angle only compliment each other, partial tooth reengagement can occur under certain circumstances, and such partial reengagement is undesirable.
It would be desirable to provide a clutch assembly which is not suspectable to partial reengagement, and which facilitates full ratcheting reengagement at higher rotational speeds during restart after flame-out by allowing the reengagement balls to positively contribute to the reengaging force and to force the disengaging balls out of their extreme positions, so that the starter turbine clutch member can move to a ratcheting reengagement position at a lower starter turbine speed.